Blender cycles rendering layer but keep to sum memory?
This look like bug and all what you can do is report as bug with most accurate info as you can. 

I noticed that Blender have problem with VRAM freeing sometimes. Try save every render layer separately. I recommend use multilayered .exr for keep render passes. Now you can composite everything in new .blend file. Optionally you can use other compositor like Natron or Fuse.

Last time I always use this workflow for more complicated projects. This method give me possibility easily rerender only layers I need later and without rendering everything again. 

I don't thing this is "stressfull way", ultimately this can save you a lot of time. Only you must link your images smartly and work will go fast :-)